Exemplary types of knife handles to which the present invention is particularly related are disclosed in U.S. Pat. Nos. 4,242,795 and 4,586,256. Such utility knives incorporate a blade carrier which mounts and supports a blade at the interior of the handle for longitudinal movement therein. The blade carrier incorporates a thumb-actuated button that can be depressed to unlatch the carrier from one of several latch positions for shifting the blade via the carrier to another longitudinal position. The thumb-actuated button may extend through a slot in the top of the knife handle and is secured to the blade carrier by means of a resilient finger. The knife handle is further adapted so that the blade may be relatively easily dismounted and replaced. The blades are replaced through the front opening of the knife handle or by gaining access through the side of the knife handle. Such knives have found great popularity and are of great utility in a wide variety of cutting operations.
While the foregoing knife handles are relatively inexpensive to manufacture and generally operate in an efficient and safe manner, a continuing concern is that during heavy use the blade may disengage from the carrier and either jamb in the knife handle or entirely disengage from the knife handle. More specifically, when torsional loads are applied to the exposed tip area of the blade, the blade may tend to move laterally and dislodge or disengage from the carrier. Disengagement of the knife blade is a particular problem when the knife is being used for cutting openings in plasterboard or drywall to accommodate junction boxes for electrical outlets, wall switches and telephone jacks and openings for heat registers and cold air returns. The corners for the openings may be cut using a twisting motion of the knife. Commonly, when using such knives, the blade may disengage or partially disengage from the blade carrier upon the application of only a low torsional force particularly when the knife is twisted in the clockwise direction. Therefore, the present invention is directed to further insuring that the knife blade is retained on the carrier and does not disengage from the carrier during use, especially when torsional loads are applied to the knife blade.